Egg removal by cuckoos forces hosts to accept parasite eggs

Abstract

Many avian brood parasites remove one or more host eggs before laying their own eggs in the host nest. Various hypotheses have been proposed to explain the adaptive significance of this behaviour, but none of them provides an adequate explanation. Here we provide a new hypothesis for explaining why a parasite removes host eggs before laying its own. In this study, we attempted to answer this question by constructing a mathematical model that focused on the changes in host decision making according to reduced clutch size as a consequence of egg removal by parasites. We assume that a host selects one of the following two options to maximise the number of its own chicks: trying to eject a suspicious egg from the nest (trying‐to‐eject) or acceptance without trying to eject the egg (acceptance). The option selected depends on the number of eggs in the nest. Our model provides a new explanation for egg removal behaviour by showing that the host should select trying‐to‐eject if there is a large number of eggs in the nest but acceptance with a small number of eggs. This is because the relative payoff for a host that selects trying‐to‐eject decreases with the number of eggs in the nest. Therefore, parasites benefit by removing the host egg because this behaviour reduces the number of eggs in the nest, thereby increasing the probability of their own eggs being accepted. Thus, hosts have evolved egg ejection to combat brood parasites, but it may also have facilitated the evolution of egg removal by parasites. This hypothesis may also apply to brood parasitic species that do not eject host chicks. In addition, this hypothesis may explain other parasitic behaviours, such as egg damaging and egg puncturing, which lead to reductions in the host clutch size.